Modularly designed polymeric materials can be engineered to suit a broad range of applications representing an attractive platform for technological advancement. Materials that possess both inherent compositional versatility and ready accessibility via robust and scalable synthetic pathways are of particular import to the field (Hunt et al. 2011, Leibfarth et al. 2010). In this regard, cationic polyelectrolytes have emerged as a versatile class of materials that have been exploited in a broad array of applications (Lodge 2008, Gao et al. 2012, Hallinan et al. 2013), ranging from gene delivery (De Smedt et al. 2000, Samal et al. 2012) to ion-conducting membranes (Pan et al. 2011, Hickner et al. 2013, Chen et al. 2010), and water purification (Elimelech et al 2011, Gin et al. 2011). Development in the area of cationic polyelectrolytes has thus far focused on a limited menu of monomeric functionalities, including ammonium, phosphonium, imidazolium, pyridinium and guanidinium ions (Jangu et al. 2014, Qiu et al. 2012, Yuan et al. 2013). These heteroatomic systems, while valuable, are application specific and suffer from the ability to finely tune their physical properties. Thus, the identification of new modular cationic polyelectrolytes, with superior characteristics for processing, controllable self-assembly and function, represents an important goal for this field (Sing et al. 2014, Steele et al. 2001). In developing a new family of polyelectrolytes, certain criteria must be met (Chen et al. 2010, Gin et al. 2011, Sing et al. 2014) including: (1) thermodynamic stability; (2) ease and scalability of polymerisations by controlled methods; (3) incorporation of accessible chemical handles to allow for diversity and intimate control of physical properties and (4) tunable Coulombic interactions. As an outgrowth of ongoing research efforts, it was postulated that polyelectrolytes based on the cyclopropenium ion could satisfy these design criteria, while offering a highly distinct structural architecture and electronic properties. It was further recognized that such cyclopropenium-based systems possess unique characteristics that distinguish them from existing cationic polyelectrolytes, namely: enhanced dispersion of the positive charge (compared with ammonium, phosphonium and guanidinium systems) and weaker H-bond donor capacity (compared with imidazolium and pyridinium ions) (Curnow et al. 2011).
As the smallest of the Hückel aromatics (Hückel 1938), the cyclopropenium (CP) ion possesses significant stability despite its carbocationic nature (Breslow 1957, Bandar et al. 2013b). This remarkable degree of stability may be further enhanced through the incorporation of amino substituents onto the CP ring (Yoshida et al (1971). Indeed, with pKR+ values estimated at >13, aminocyclopropenium ions are stable even in strongly alkaline aqueous solutions (Yoshida et al. 1974, Kerber et al. 1973). Moreover, thermal decomposition (Tdec) of the tris(dialkylamino)CP chloride salts has been measured at >300° C. (Curnow et al. 2011), significantly exceeding that of dialkylimidazolium chloride salts (Tdec, 250° C.) (Huddleston et al. 2001). These unique structural features have already inspired the development of aminocyclopropenium ions for a range of applications, including as metal ligands (Bruns et al. 2010), organocatalysts (Bander et al. 2012, Bander et al. 2013, Wilde et al. 2013) and ionic liquids (Curnow et al, 2011); however, the incorporation of these cations into a polymeric backbone has only led to polymers with unstable CP ions as intermediates (Weidner et al. 1995). Given the tunable functionality and robust, efficient and orthogonal chemistry characterizing CP ions, it is desirable to exploit them in polymeric materials.
Indeed, the ability to incorporate stable ionic moieties on linear, branched, dendritic, and cross-linked polymeric systems has led to the development of materials that can be employed in a wide variety of applications, such as water purification, drug delivery, gene therapy, antimicrobial coatings, ion transporting membranes, and as cell substrates, among others. For example, water desalination membranes are currently being synthesized by cross-linking polymerization of 1,3-benzenediamine and trimesoyl chloride, to yield a polyelectrolyte. Other materials, such as electrostatic layers have also been evaluated. Starpharma has developed dendritic polyelectrolytes based on polyamidoamine (PAMAM) as HIV prevention drugs and for drug delivery. Drug delivery vectors containing guanidine have also been shown to be effective mimics of cell-penetrating peptides.
Such polymers are desirable for many reasons, but available materials suffer from a number of limitations, such as pH sensitivity, difficulty of synthesis, or lack of variability. For example, in water purification membranes, currently available materials lack tunable mechanical properties and can be brittle. Furthermore, the chemistry is more difficult to manage due to the fact that the acid chlorides that are currently used are water sensitive, and must be processed in dry conditions.
Accordingly, there is a need for, inter alia, stable ionic moieties on linear, branched, dendritic, and cross-linked polymeric systems that are simple to prepare, are broadly tunable in terms of their properties, and are stable across a wide range of pH levels. The present invention is directed to meeting these and other needs.